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Hydrogen bonding in (COOH)2 • 2H2O and (COOD)2• 2D2O

Despite repeated efforts, vibrational spectra of single crystal oxalic acid are still not assigned properly in all spectral intervals studied. Far infrared study of single crystals of (COOH)2• 2H2O by Wyncke, Brehat and Hadni gave the assignment of infrared active phonons and internal vibrations with frequencies below 200 cm-1 [1], while Villepin and Novak concentrated on vibrations involving hydrogen bond and did not present the list of all observed frequencies [2]. Ebisuzaki and Angel compared Raman bands of two polymorphs of oxalic acid dihydrate, α and β , both belonging to P21/c space group and with two molecules per unit cell [3]. Their assignment was empirical and some bands were left unassigned. We have recorded Raman spectra of single crystals of α (COOH)2• 2H2O and α (COOD)2• 2D2O, including low temperature Raman spectra of α (COOH)2• 2H2O, and compared the positions of observed bands with the values calculated with CRYSTAL06 and Car-Parrinello molecular dynamics programs. The values given by CRYSTAL06 program altogether are in better agreement with the observed positions of phonons, but the value of carboxyl O-H stretching frequency is still too high (calculated values are 2289 cm-1 for Ag and 2543 cm-1 for Bg mode respectively, while the observed Raman band is at 1959 cm-1). For deuterated sample, the carboxyl ν (O-H) appears at practically the same place, at 1938 cm-1, indicating strong hydrogen bond. The other modes relating to OH carboxyl group are attributed to γ (OH) at 1265 cm-1 (infrared band), while δ (COH) is strongly mixed with H2O bending modes (1415 cm-1, 1439 cm-1, 1530 cm-1, 1635 cm-1 are observed Raman bands). Recently , two new crystal forms of sesquihydrates of oxalic acid were reported [4]. One crystal structure belongs to P1ˉ group with two molecules per unit cell, while the other belongs to Pnma with eight molecules per unit cell. Compared to α (COOH)2• 2H2O, which has the closest O · · · O distance equal to 2.484 &Aring ; , and that is between O(H) of carboxyl group and O from water, in the P1ˉ group the closest O· · · O distance between the same two groups is 2.519 &Aring ; , while the next one is equal to 2.505 &Aring ; and involves the distance between hydroxyl oxygen from COOH group of the first molecule and carbonyl oxygen from the COO- group of the second molecule. The way water is incorporated between oxalic acid molecules could explain the fact that in some Raman spectra three bands were observed where only two were expected for ν (C-C) stretching band.

strong hydrogen bond; oxalic acid; Raman spectra of single crystals

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